test: regression test for uv threadpool congestion

Validate that massive dns lookups do not block filesytem I/O
(or any fast I/O for that matter).
Prior to https://github.com/libuv/libuv/pull/1845 few back-to-back dns
lookup were sufficient to engage libuv threadpool workers in a blocking
manner, throttling other work items that need the pool. this test acts
as a regression test for the same.

Start slow and fast I/Os together, and make sure fast I/O can complete
in at least in 1/100th of time for slow I/O.

Refs: https://github.com/libuv/libuv/pull/1845
Refs: https://github.com/nodejs/node/issues/8436

PR-URL: https://github.com/nodejs/node/pull/23099
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/internet/test-uv-threadpool-schedule.js

@@ -0,0 +1,65 @@
+'use strict';
+
+// Test to validate massive dns lookups do not block filesytem I/O
+// (or any fast I/O). Prior to https://github.com/libuv/libuv/pull/1845
+// few back-to-back dns lookups were sufficient to engage libuv
+// threadpool workers in a blocking manner, throttling other work items
+// that need pool resources. Start slow and fast I/Os together, make sure
+// fast I/O can complete in at least in 1/100th of time for slow I/O.
+// TEST TIME TO COMPLETION: ~5 seconds.
+
+const common = require('../common');
+const dns = require('dns');
+const fs = require('fs');
+const assert = require('assert');
+
+const start = Date.now();
+
+const slowIOmax = 100;
+let slowIOcount = 0;
+let fastIOdone = false;
+let slowIOend, fastIOend;
+
+function onResolve() {
+  slowIOcount++;
+  if (slowIOcount === slowIOmax) {
+    slowIOend = Date.now();
+
+    // Conservative expectation: finish disc I/O
+    // at least by when the net I/O completes.
+    assert.ok(fastIOdone,
+              'fast I/O was throttled due to threadpool congestion.');
+
+    // More realistic expectation: finish disc I/O at least within
+    // a time duration that is 1/100th of net I/O.
+    // Ideally the slow I/O should not affect the fast I/O as those
+    // have two different thread-pool buckets. However, this could be
+    // highly load / platform dependent, so don't be very greedy.
+    const fastIOtime = fastIOend - start;
+    const slowIOtime = slowIOend - start;
+    const expectedMax = slowIOtime / 100;
+    assert.ok(fastIOtime < expectedMax,
+              'fast I/O took longer to complete, ' +
+              `actual: ${fastIOtime}, expected: ${expectedMax}`);
+  }
+}
+
+
+for (let i = 0; i < slowIOmax; i++) {
+  // We need to refresh the domain string everytime,
+  // otherwise the TCP stack that cache the previous lookup
+  // returns result from memory, breaking all our Math.
+  dns.lookup(`${randomDomain()}.com`, {}, common.mustCall(onResolve));
+}
+
+fs.readFile(__filename, common.mustCall(() => {
+  fastIOend = Date.now();
+  fastIOdone = true;
+}));
+
+function randomDomain() {
+  const d = Buffer.alloc(10);
+  for (let i = 0; i < 10; i++)
+    d[i] = 97 + (Math.round(Math.random() * 13247)) % 26;
+  return d.toString();
+}